Heat-resistant members of aircraft engines, power-generating gas turbines, etc., for example, turbine disks are parts holding rotor blades and rotating at a high speed and require a material which can withstand a very high centrifugal stress and is excellent in fatigue strength, creep strength and fracture toughness. On the other hand, an improvement in fuel consumption and performance calls for an improvement in engine gas temperature and a reduction in weight of turbine disks and thereby requires a material of still higher heat resistance and strength.
Nickel-based forged alloys are generally employed for turbine disks. For example, there are widely used Inconel 718 having a γ″ (gamma double prime) phase as a strengthening phase and Waspaloy having as a strengthening phase about 25% by volume of a precipitated γ′ (gamma prime) phase which is more stable than the γ″ phase.
In view of a tendency toward a higher temperature, Udimet720 which had been developed by Special Metals was introduced in 1986. Udimet720 is an alloy having about 45% by volume of a precipitated γ′ phase, containing tungsten to strengthen the solid solution of the γ′ phase and having a particularly excellent heat-resistant property. However, as a TCP (topologically close packed) phase which is low in structural stability and harmful is formed in Udimet720 during its use, Udimit720Li (U720Li/U720LI) improved by e.g. a reduction of chromium was developed. However, a TCP phase is formed in Udimit720Li, too, and restricts its use for a long time or at a high temperature. It is also pointed out that Udimit720 and 720Li have a narrow process window for e.g. hot working or heat treatment because of a small difference between their γ′ solidus temperature (solvus) and initial melting temperature. Accordingly, it is a practical problem that the manufacture of a homogeneous turbine disk by a casting and forging process is difficult.
Powder metallurgical alloys, such as AF115, N18 and Rene88DT, are sometimes used for high-pressure turbine disks of which high strength is required. The powder metallurgical alloys have the advantage of being able to make homogeneous disks having no segregation, even though they contain many strengthening elements. On the other hand, a high level of control of the manufacturing process, including vacuum melting with high purity and the selection of a proper mesh size for powder classification, is required to prevent the mixing of inclusions and presents a problem of cost increase.
Numerous proposals have hitherto been made for improvements in the chemical compositions of nickel-based heat-resistant superalloys, and all of them contain cobalt, chromium, molybdenum or molybdenum and tungsten, aluminum and titanium as their principal constituent elements, and typical ones contain niobium or tantalum or both as their essential constituents. In the composition as described, the presence of niobium and tantalum is suitable for powder metallurgy as described above, but is a factor making casting and forging difficult. Cobalt is contained in a relatively high proportion, but JP-A-10-46278 of the application by Rolls Royce, for example, states that it does not produce any particularly significant result, and while it is generally considered to bring about positive results by realizing a lower γ′ solidus temperature and a widened process window, EP 1 195 446 A1 of the application by General Electric Company does not show any other result, but limits its content to 23% by weight or less by considering cost, etc., too.
On the other hand, titanium is added as it serves to strengthen the γ′ phase and thereby improve tensile strength and crack propagation resistance. However, it is limited to, say, 5% by weight, since the excessive addition of titanium results in a higher γ′ solidus and a harmful phase formed to disable the formation of a sound γ′ structure.
Therefore, it is difficult for the existing art to provide a heat-resistant superalloy which can withstand a long time of use at a high temperature, permits casting and forging, and is very easy to manufacture.